The present invention relates to a photographic lens, particularly of the Gauss type.
Further, the present invention relates to a photographic lens presenting a long back focal distance suitable for a single lens reflex camera. The photographic lenses of the Gauss type are widely used, whereby the British Pat. No. 1,120,335 discloses such a photographic lens whose cemented doublet lens immediately before the diaphragm is put apart so as to provide an air gap between the positive meniscus lens and the negative meniscus lens. The British Patent was published on July 17, 1968.
Ever since, the photographic lens of the Gauss type whose doublet lens is put apart has been improved and modified in various ways, whereby the recent ones are disclosed for example, with the U.S. Pat. Nos. 3,938,884 and 3,984,155.
In the case of the photographic lens with the long back focal distance on the one hand, it is necessary to prepare the refractive power of the lens provided behind the diaphragm disposed in the air gap between the two negative doublet lenses remarkably larger than that of the lens provided in front of the diaphragm. In consequence, in order to realize a compact lens, keeping the back focal distance, it is necessary to increase the radius of curvature or the number of the lenses in order to support the strong refractive power of the lens part behind the diaphragm, whereby it becomes difficult to obtain a compact lens system whose total length is decreased. Further, the refractive power before the diaphragm is different from that behind the diaphragm and therefore the symmetricity with reference to the diaphragm as center is remarkably destroyed in such a manner that it is considerably difficult to obtain a proper aberration compensation of a lens with a large aperture.
Hereby, in order to obtain a compact lens and a proper aberration compensation, the refractive power of the lens part before the diaphragm has to be also increased, whereby on the other hand, the radius of curvature of each lens is enlarged in such a manner that it becomes also the more difficult to compensate various aberrations satisfactorily.